In recent years, hybrid vehicles and electric vehicles have receives attention as vehicles developed in view of environments. The hybrid vehicle uses, as drive power sources, a DC power supply, an inverter and an electric motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to provide the drive power source, and further the inverter converts a DC voltage provided from the DC power supply into an AC voltage for driving the motor by the converted AC motor to provide the drive power source.
The electric vehicle uses, as drive power sources, a DC power supply and an inverter as well as an electric motor driven by the inverter.
For appropriately driving the vehicle and improving energy efficiency, the hybrid vehicle or the electric vehicle described above is required to supply an electric power corresponding to a load to the motor and to recover energy efficiently in a regenerative operation.
For satisfying the above requirements, e.g., Japanese Patent Laying-Open No. 2004-15866 has disclosed a hybrid vehicle in which an electric power source of an electric motor is formed of a power storage device including a secondary battery and a capacitor connected in parallel.
This structure controls charge/discharge of the power storage device to attain its residual capacity that maximizes a quantity of generated heat of the secondary battery. Therefore, even when the power storage device is to be charged with a constant electric power, the temperature of the secondary battery can be rapidly increased to improve a dischargeable output and a chargeable input of the power storage device.
Further, Japanese Patent Laying-Open No. 2004-312926 has disclosed a control device of a vehicle for driving an electric motor by supplying an electric power from a capacitor or via an electric power converter circuit from a secondary battery.
The above control device of the vehicle includes control means for controlling an operation of an electric power converter circuit according to a state of a load on the electric motor. When the control means does not operate the electric power converter circuit according to the state of the load, the capacitor can supply the electric power to the electric motor. Therefore, an energy loss does not occur in the electric power converter circuit, and fuel consumption can be improved. When the control means operates the electric power converter circuit, the secondary battery can supply the electric power so that the electric motor can generate a sufficient drive power for driving the vehicle, and lowering of the vehicle drive performance can be prevented.
In the control device of the vehicle that is provided with the capacitor and the secondary battery as the power supply sources for the electric motor as described in the above patent documents, electric charges remaining in the capacitor may be discharged after a vehicle system stops. For example, in a maintenance operation of the power supply device, the charges remaining in the capacitor are discharged before performing the maintenance for ensuring operation safety.
Therefore, in a next start operation of the vehicle system, the capacitor is in an overdischarged state, and will be charged by the battery when the converter operates when a driver turns on an ignition.
In the above state, however, the voltage of the capacitor is substantially zero so that an excessive current (i.e., rush current) may flow into a capacitor due to a voltage difference with respect to the battery. This rush current may internally overheat and damage the capacitor, and may weld a relay employed for connecting the capacitor to a power supply line. This results in a problem that the vehicle system cannot be started up immediately after the discharge of the capacitor, in view of generation of the above rush current.
For avoiding the rush current, it is effective to arrange a current limiting device for controlling a charge/discharge current of the capacitor. The current limiting device may be a resistance or a reactor.
In the control device of the vehicle using the capacitor of a large capacity as the electric power source, however, the capacitor itself has a high output density so that the current limiting device for it must have a high impedance and large sizes. This may increase the sizes of the device.
Accordingly, the invention has been made for overcoming the above problems, and an object thereof is to provide a power supply device that can be implemented by a small device structure with high safety as well as a control method of the power supply device.